Lecture Notes 12.001 Magmatic Rocks
O.Jagoutz
Magmatic rocks. If we want to understand the origin and evolution of magmatic
rocks we need to know two things: how does the Earth melt and how does the melt
evolve (differentiate)?
Part I
How does the Earth melt?
Show the topographic map and ask students which volcanoes they know. Ask them
why they think that these volcanoes are there. Point out ridges, subduction zones
and hotspots. Discuss a bit the characteristics of volcanic eruptions, why do we
enjoy looking at an eruption in Hawaii whereas Mt St. Helens and Krakatau weren’t
really very pleasant for the people that experienced it.
Courtesy of Rob Butler. Used with permission.
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Courtesy of Rob Butler. Used with permission.
Show mantle phase diagram (Langmuir et al. ), show solidus and liquidus and
indicate the location of the Earth’s mantle in P,T space. Ask how we can melt the
mantle? Ask if they think that we can ever melt the mantle completely? Probably not
- an exception could be extremely high energy events like meteorite impacts.
Show the different mechanisms by which we could melt the mantle: decompression,
heating and flux melting. Draw in the hydrous solidus - indicate that this is the
temperature at which the mantle melts when it’s saturated with H2O. Explain the
difficulty we have experimentally in locating the hydrous solidus and that estimates
range from ~ 750 -800 C to 1100C.
Explain the concept of mantle potential temperature and the adiabat, explain that
the slope of an adiabat is due to thermal expansivity and heat capacity.
T(z) =T0 exp(alpha*g/Cp * (z-z0)
To account for the T change with decreasing pressure after we crossed the solidus
we have to include heat of fusion terms in the eq, which we don’t do here. But this is
what changes the slope as we lose energy to the melting process.
Explain how the degree of melting varies with different melting temperatures, and
use this to introduce the discussion that this might have changed through time. That
determining the degree of partial melting of melts is a possible tool to constrain the
thermal state of the upper mantle and that we can use the geological record to
constrain the thermal evolution of the earth. Explain that an important parameter is
MgO content of these primary melts, which is thought to relate to degree of melting
and therefore temperature. And people try to constrain the change in MgO content
through time. Ask them why this is difficult. Explain that degree of melting is also
composition-dependent and, as we have seen, water changes the story completely.
So connecting degree of melting to temperature is possible but involves a lot of
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assumptions. Draw a diagram of MgO content and time and that we try to relate this
to temperature.
Discuss the different geodynamic settings in which we can melt the mantle. Show
ridges. Subduction volcanism and hotspots.
Part II
What happens to the melt? It either gets stuck somewhere in the crust or erupts on
the surface. What is the profound difference for the rock? Crystal size. Aim at
understanding the temperature difference the melts have to adapt too. Than draw
the diagram of nucleation rate vs. growth rate.
How does melt differentiate?
So now that we formed a melt and we extracted it from the mantle it will encounter
a decreasing temperature gradient. What will happen? Let’s do a thought
experiment - we take our graduate student slave which in our case is our beloved TA
Ben and we send him to two places in the world. We send him to dive down to the
Oceanic ridges, and we send him to Oregon to sample magmatic rocks from the
different volcanoes. So when he comes back we treat him nicely and we send him
into the lab and he does chemical analyses for us. So than we sit down and plot the
results and we realize a profound difference between the rocks we analyzed.
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Explain calc-alkaline (effect of water - arcs) and tholeiites (no water – mid-ocean
ridges)
Draw Tim’s diagram and explain the concept of crystal fractionation. Draw a
schematic magma chamber and explain cumulates and liquid separation. This
results in chemical differentiation.
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Point out the importance of textures and draw a schematic diagram of a rocks that
follows the high P vs. low P fractionation.
Say that this has long been recognized – initially by a guy named Bowen who was a
student here at MIT. The relations he noticed are now called Bowen’s reaction series.
Courtesy of Mike Strickler. Used with permission.
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12.001 Introduction to Geology
Fall 2013
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